1. Technical Field of the Invention
The present invention relates generally to rotating electrical machines, such as electric generators and motors. More particularly, the invention relates to a rotor for an automotive alternator which includes an improved magnet holder for holding a plurality of permanents magnets between interleaved claws of lundell-type pole cores.
2. Description of the Related Art
There is known a method of improving the efficiency of an automotive alternator, which includes a rotor having a pair of lundell-type pole cores, by disposing a plurality of permanent magnets between the pole cores.
More specifically, each of the lundell-type pole cores is secured on a rotary shaft of the rotor and includes a plurality of claws extending in the axial direction of the rotary shaft. The claws of one of the pole cores are interleaved with those of the other pole core. Further, according to the method, each of the permanent magnets is interposed between adjacent two of the claws of the pole cores in the circumferential direction of the rotary shaft, so as to diminish the magnetic flux leakage between the adjacent two of the claws and to direct the magnetic flux developed by the permanent magnet toward a field coil of the alternator, thereby improving the efficiency of the alternator.
Moreover, U.S. Pat. No. 5,925,964 discloses the use of a resin-molded magnet holder for securely holding the permanent magnets between the claws of the pole cores.
According to the disclosure, the magnet holder is shaped in a meandering ring that extends in the circumferential direction in a zigzag manner. The magnet holder includes a plurality of board parts (i.e., holding portions), each of which axially extends to hold thereon one of the permanent magnets, and connecting parts (i.e., connecting portions) each of which circumferentially extends to connect middle portions of adjacent two of the board parts.
However, for formation of the magnet holder by resin molding, a complicated mold is required due to the complexity of shape of the magnet holder, thus increasing the manufacturing cost.
Further, to secure the strength of the magnet holder and to prevent formation of underfill during the resin moldering process, it is necessary to make the thickness of the magnet holder being above a certain level, for example 1 mm.
However, with such a thickness of the magnet holder, since the connecting parts connect the middle portions of the board pats and are thus radially adjacent to the field coil, the space available for winding the field coil around the pole cores is accordingly reduced. Consequently, the strength of the magnetic field is lowered, thus reducing the power output of the alternator.
Furthermore, in the resin molding process, a mold release agent is generally applied on the mold so as to make it easy to release a work from the mold.
However, the mold release agent usually adheres to the work and thus remains on the surface of the resultant magnet holder. Consequently, in fitting the magnet holder between the claws of the pole cores by adhesion, it is difficult to secure sufficient adhesion strength.
In addition, since the magnet holder is made of a resin material, it may be damaged during assembly of the rotor due to an excessive force applied thereon.
On the other hand, Japanese Patent First Publication No. H10-66286 discloses the use of a protective cover for each of the permanent magnets.
According to the disclosure, the protective cover is made of a resin or metal material. The permanent magnet covered with the protective cover is press-fitted between circumferentially adjacent two of the claws of the pole cores.
However, since each of the permanent magnets is covered with an individual protective cover, the number of the protective covers is the same as that of the permanent magnets and it is required to fit the permanent magnets between the claws of the pole cores in a one-by-one manner. Consequently, both the parts count and the steps of assembly of the rotor are increased, thus increasing the manufacturing cost.
Further, to securely fix the permanent magnets, the circumferential side faces of the claws of the pole cores are toothed to bite the respective protective covers. Consequently, due to the additional process of toothing, the manufacturing cost is further increased.